1. Field of the Invention
The present invention relates to an improvement on a MOS gate controlled thyristor (to be referred to as an MCT hereinafter) which can be turned on/off using a MOS gate.
2. Description of the Related Art
FIG. 1 shows the structure of an MCT described in copending application filed Oct. 30, 1992, Ser. No. 969,491. The steps in manufacturing the MCT will be briefly described below. An n.sup.+ -type buffer layer 6 and a lower-surface p.sup.+ -type emitter layer 7 are formed on the lower surface of an n.sup.- -type semiconductor substrate 1. A gate oxide film 8 and a polysilicon gate electrode 9 are formed on the upper surface of the substrate 1. A p-type base region 2, p.sup.- -type base regions 3, a p-type source region 4, and an n-type emitter region 5 are respectively formed in the surface regions of the substrate by diffusion. In addition, an opening is partially formed in the gate oxide film 8 to form cathode electrodes 10. An anode electrode 11 is formed on the lower surface of the substrate 1.
An operational principle (turn-on/turn-off operations) of the MCT will be described below. Note that FIGS. 2A to 2C are views for explaining a turn-on operation, and FIGS. 3A to 3C are views for explaining a turn-off operation. FIGS. 2A and 3A are plan views; FIGS. 2B and 3B are sectional views along an Y-Y' line in FIG. 1; and FIGS. 2C and 3C are sectional views along a X-X' line in FIG. 1.
The turn-on operation will be described below with reference to FIG. 2A to 2C. An anode is biased positive, cathodes are biased negative, and a gate is biased positive. In this state, n-channel inversion layers 12 are formed in the p.sup.- -type base regions 3, and electrons are injected from the n-type emitter region 5 into the n.sup.- -type base regions (substrate) 1. In this manner, holes 14 are induced from the lower-surface p.sup.+ -type emitter layer 7, and the n.sup.- -type base region 1 is subjected to conductivity modulation. Portions 15 where the p.sup.- -type base regions 3 is in contact with the n-type emitter region 5 become early turn-on areas, thereby starting the turn-on operation of the device. When a turn-on region extends over the entire area of the n-type emitter region 5, the device is completely turned on.
The turn-off operation will be described below with reference to FIGS. 3A to 3C and 4. When the anode is biased positive, and the cathodes are biased negative, a main current 17 flows. In this state, when the gate is biased negative, the n-channel inversion layers 12 which are formed during the turn-on operation disappear. A p-channel inversion layer 16 is formed on the surface of the n.sup.- -type base region 1 between the p-type base region 2 and the p-type source region 4. The p-type base region 2, the p-type source region 4, and the cathode electrodes 10 are short-circuited to each other, and holes 18 of the main current are discharged from the cathode electrodes 10. With this operation, injection of electrons from the n-type emitter region 5 is stopped, and the main current 17 does not flow. In the n-type emitter region 5, the turn-off operation is started from a region 19 opposite to the p-type source region 4. Finally, the turn-off region extends over the n-type emitter region 5, thereby completing the turn-off operation.
A MCT has been developed as a self-turn-off device to improve its turn-off efficiency having priority over other conditions. In this technique, an on-gate portion is separated from an off-gate portion, and a ratio of the area of the off-gate portion to the area of the on-gate portion is set such that most of the area of the gate is used as the off-gate portion. In addition, in portions except for the on-gate portion, in order to improve turn-off characteristics, the concentration of the p-type base region is increased, and a method of decreasing the resistance of a path, formed in a turn-off operation and constituted by the p-type base region 2, the p-channel inversion layer 16, and the p-type source region 4, for discharging a hole current is used.
According to this method, the turn-off region in the on-gate portion has a low impurity concentration due to the p.sup.- -type base region 3. For this reason, although the turn-off operation of the off-gate portion can be preferably performed, a short-circuit resistance is not decreased, and a hole current is not sufficiently discharged. That is, the turn-off operation of the on-gate portion is not efficiently performed.
On the other hand, in the turn-on operation, even when electrons are injected from the on-gate portion into the n.sup.- -type base region (substrate) 1, electrons are not easily injected from the n-type emitter region 5 except for the on-gate portion. The turn-on operation is performed by causing the on-gate region to extend over the entire area of the n-type emitter region 5. Therefore, in the semiconductor device described above a small on-gate area, the efficiency of the turn-on characteristics is poorer than that of the turn-off characteristics, and trade-off between the turn-on characteristics and the turn-off characteristics cannot be easily obtained.
As described in the above technique, a turn-off operation of the on-gate portion is not efficiently performed. On the other hand, the efficiency of the turn-on characteristics is poorer than that of the turn-off characteristics, and trade-off between the turn-on characteristics and the turn-off characteristics cannot be easily obtained.